This invention relates generally to a guardrail structure mounted along a roadway, and more particularly to post supports for supporting guardrail beams or panels which extend longitudinally in a direction generally parallel to the roadway.
Roadway safety barrier and crash attenuation systems are an important safety feature and component of today""s roadways. These systems serve to address the potentially catastrophic results of situations where errant motorists might otherwise leave the relative safety of the designated roadway, or might stray from the safety of normal traffic conventions. They accomplish this by redirecting the vehicle away from a hazardous area in a controlled manner, while absorbing some of the energy of the vehicle through deformation of the system. These systems often include portions having posts that serve as an integral component. This is because posts contribute to the effectiveness, economy of manufacture, ease of installation, and maintenance of these systems, as well as to their reliability.
The posts of a typical safety barrier or crash attenuation system serve to maintain the system in its optimal configuration and state of readiness relative to the roadway, including factors such as height, spacing, support, tension, rigidity, and energy absorption capability. These configuration aspects enable the various components of the system to perform in unison to accomplish their overall purpose of protecting motorists by absorbing and dissipating energy as the system reacts and deforms while responding to errant vehicles. In these applications, the posts are commonly fastened (bolted) or welded to various other roadway features, and may also be partially submerged in the ground in order to give them rigidity as well as to provide a means of anchoring the system while transmitting impact forces to the ground.
Over the past several years, the Federal Highway Administration (FHWA) as well as State Departments of Transportation (TDOT""s) throughout the country have increasingly sought to improve the economy, strength, and effectiveness of roadway barrier and crash attenuation systems, including posts, guardrails, fasteners, end treatments, and other components. Thus, installed systems and their components have been required in recent years to sustain increasingly higher levels of economy and performance. This has led to system test requirements that reflect these increasingly higher standards.
Accordingly, these systems are now commonly tested using vehicles having somewhat increased speeds and angles of incidence upon impact with the systems. However, these seemingly small changes in vehicle speed and trajectory may result in substantial increases in the performance requirements of the system. This is because the forces that are imposed upon a system and its components during an impact are highly sensitive to vehicle speed and angle of incidence. Moreover, still additional increases in system forces have been introduced as follows. First, typical test vehicles now have increased mass. Second, the types of test vehicles have been modified to more adequately represent the actual fleet of vehicles on today""s roadways. These modifications include vehicles having higher bumpers and centers of gravity, both of which contribute to greater challenges for barrier and crash attenuation systems in achieving-successful performance.
This trend toward increasing economy and performance is very desirable. Yet is has imposed a great challenge on the roadway safety community, because these specifications sometimes seem to require conflicting characteristics of the system. The discussion below describes several aspects of this challenge, with particular emphasis on the implications for existing conventional barrier post designs and the need for innovations that can adequately address the shortcomings of the present state-of-the-art in a cost-effective manner.
The first and most common approach taken by the roadway safety community in addressing these higher requirements has been to make the conventional barriers and posts out of heavier gauge material. For example, heavier gauges of guardrail, made from 0.130 inch thick (10 gauge) material now seem to be more common. I-beam posts are sometimes specified in weights of eight and a half and more pounds of steel per foot. This corresponds to specified flange thicknesses of 0.194 inches, and web thicknesses of 0.170 inches for a W6xc3x978.5 post. In some applications even heavier I-beam posts are used. The use of thicker material has not only led to greater cost for roadway product manufacturers and consumers but also, as will be shown, has had the effect of creating other challenges simultaneously. The following is a discussion of various aspects of these challenges.
The approach of simply increasing material thickness in order to address higher standards may initially seem to minimize the number of changes that are required in updating specific parts of the system, such as the posts. However, this approach may also have some consequences in terms of its effect upon the vehicle. This is because thicker, heavier, and more rigid barrier systems may impose more sudden changes upon the trajectory and speed of an errant vehicle that can in turn affect the vehicle occupants. In addition to this, as the posts are made to be heavier, the posts themselves may become significant obstacles and sources of undesirable local levels of impact to the occupant compartment of the vehicle.
Moreover, in some barrier systems such as guardrail systems, the heavier posts may represent such an obstacle that they often inherently include or otherwise incorporate special characteristics that give them directional strength. This makes them stronger in one direction as compared with the transverse direction with respect to the roadway. One example of this is found in some longitudinal barriers having discontinuities or terminations near their longitudinal ends. In such cases it is often desirable to permit some of the end terminal posts to selectively break away or collapse to the ground rather than represent an obstacle that might unduly damage the vehicle if the end terminal region is struck head-on by a vehicle.
But the increase mass of barrier posts are not the only challenge facing the present state-of-the-art for barrier and crash attenuation system posts. The following is a discussion of some additional considerations that need to be addressed. In this discussion, specific geometrical features are discussed along with their performance characteristics.
Several types of posts are commonly used today in roadway barrier systems. One very common type of post that is found in roadway barrier systems is made of wood. These posts may be of round or rectangular cross-section. Others are hybrids that are made of metals such as steel in combination with materials such as wood or plastic. Hybrid posts are not considered to be extremely viable because of processing costs, and because of complexities associated with maintaining strong and viable interfaces between the materials over extended periods of sunlight, moisture, and temperature cycling during service.
Steel posts include those that have sections that are hot-rolled, cold rolled, or xe2x80x9cbuilt upxe2x80x9d or joined sections that may represent open or closed cross-sections. Material cost, durability, reliability, and maintenance issues have favored a trend toward steel posts over wood or hybrid systems. However, these posts have remained relatively the same over the past few decades. Cost is always a consideration as more rigid (and thus generally heavier) conventional posts are considered. Other considerations are discussed below using the common hot-rolled steel I-beam post as an example.
Hot-rolled I-beam sections have become even more popular in recent years as the price of wood has risen. These sections consist of simple flat flanges that are joined by a middle web. Guardrail panels are commonly bolted directly to the flanges, and commonly have spacers or xe2x80x9cblock-outsxe2x80x9d to hold the installed guardrail panels away from the posts in order that vehicle tires will not tend to snag on the posts as they contact the barrier system during a crash.
The hot-rolled I-beam post has found favor in the roadway safety industry because it is robust, simple, and permits easy access during assembly for tightening the nuts of the post bolts that hold the guardrail onto the posts. The flat outer surface of the I-beam provides a smooth surface onto which to mount the block-outs and guardrail panels. In addition, this simple shape is relatively easily handled and installed, either into pre-dug holes, or directly into the soil by machines that drive the post into the soil. Finally, when the post is made of steel, it tends to have somewhat greater durability in the field, than when it is made of treated wood. This advantage is especially evident in regions where rainfall, insect, and climate conditions may combine to affect the durability of wood posts.
As simple and useful as the hot-rolled I-beam post has proven to be, it still has inherent aspects that influence its economic potential for future roadway safety applications. One such consideration is that the I-beam cross-section is inherently a sufficiently stable cross-section for the present thicknesses that are manufactured, but may be less stable if thinner material is used. In service it commonly flattens toward the ground in a failure mode called xe2x80x9clateral-torsional bucklingxe2x80x9d as it experiences high loads during a crash. Buckling is a failure mode that is commonly associated with lower load levels (and thus section stress levels) than the structure is otherwise capable of sustaining. Buckling is discussed in greater detail below. A post section that buckles easily is probably not a weight-efficient design, since it tends not to take full advantage of the maximum strength of the material. Thus, thinner I-beam sections may not be likely candidates for future applications.
This means that thicker sections must commonly be used in order for the I-beam post to adequately resist the buckling failure mode, so that it may in turn provide the required level of rigidity and support to the guardrail system. Moreover, this use of thicker sections has had the effect of making the post heavier, since more material is required.
Another notable effect is that the post becomes more robust in other ways. This robustness is helpful in such cases where it is necessary and cost-efficient to drive the posts into the ground during installation, using semi-automated driving machines. However, some performance challenges have surfaced with the I-beam post that relate to its robustness and to its cross-sectional shape. When installed, I-beam posts most often present a blade edge toward the tires of oncoming vehicles that encounter the guardrail. This may result in the tire somewhat more easily snagging on the post, which may in turn impinge on the vehicle as it interacts with the guardrail. In some cases the tire may be completely separated from the vehicle during a crash as a result of snagging on a post. Naturally, this may have an additional effect upon the vehicle.
Since heavier I-beam posts have represented a mixture of advantages and disadvantages, some of which relate to overall system performance, the highway safety community has sought out alternative and more economical configurations and section shapes. However, only marginal progress has been made in this effort. Some specific challenges are discussed below.
It may be noted here that closed-section posts are not common because they lack the weight-efficiency to represent an economical solution. First, closed sections are generally not as efficient as open sections in achieving sectional properties that resist bending during an impact. In addition, closed sections often lack the ability to hold firm as they interact with the soil during an impact. In addition, closed sections often lack the ability to hold firm as they interact with the soil during an impact, thus necessitating longer sections of buried length in the soil. Finally, closed sections are generally more costly to manufacture than open sections. For these and other reasons, the remainder of this discussion will focus n open section posts.
Open section metal post configurations have included 0.170 inch thick C-shaped cross-sections with blade edges. The C-shaped cross-sections have generally been cold-formed sections that were made by roll forming. One fundamental shortcoming of C-shaped cross-sections in general has been that the blade edges along the length of the post are particularly susceptible to edge instabilities such as edge-buckling or crimping during service and installation.
Edge buckling is a characteristic problem of open section posts in bending, and is related to a free edge stress concentration. It is important because it represents a local failure mode that, having initiated at relatively low stress levels, may propagate across the entire section, causing the post to lose its capability to support the barrier, and thus to fail. This consideration has in fact been a significant driver toward the use of thicker material for most open section post configurations. As a result, these sections have not proven to be competitive because they have not been more economical than hot-rolled I-beam sections.
C-section posts have also been found to generally lack the ability to be effectively driven into the soil by mechanical means during installation. This is because the blade edge xe2x80x9ccornerxe2x80x9d lacks the support of adjacent material and is thus particularly susceptible to local bending as it contacts the soil during installation. The resulting xe2x80x9cbent earxe2x80x9d of the corner tends to act like a rudder that distorts the flanges of the section and thus the overall section shape, as the post passes through the soil.
Thus, multiple deficiencies exist for open section posts, including I-beam and C-section posts due to their blade edges. These deficiencies have resulted in roadway barrier posts that are generally more costly, yet are barely adequate to simultaneously meet important design and economic considerations. Consequently, these conventional posts hold limited promise for future economical improvement without innovations that are able to advance the state-of-the-art and to provide the best possible posts for the best possible price to consumers.
In summary, because of increasingly higher safety requirements and the challenges of economy associated with using conventional heavier gauge open section steel posts, there is a need within the industry today for a new stabilized open section metal post configuration that can substantially address all of the above-mentioned drawbacks and shortcomings of the present state-of-the-art, yet is suitable for use with substantially all standardized roadway safety hardware, and can be made on a cost-effective basis. It should also have more stable, tailorable performance characteristics, be economical to manufacture, and be able to conserve some of the desirable capabilities that steel posts offer in general.
The present invention alleviates and substantially overcomes the above-mentioned problems and shortcomings of the present state of the art through a novel roadway barrier post may be 1) is made of thinner material, 2) performs adequately to enable it to meet new higher roadway safety requirements, 3) may be significantly more resistant to edge buckling during installation and service, 4) effectively addresses edge stress concentrations by modifying the blade edge to an area of relatively low stress, 5) offers enhanced resistance to lateral-torsional buckling, 6) may be manufactured cost-effectively by using conventional manufacturing methods, and 7) may be tailorable in terms of local design characteristics that serve to significantly extend its range of adaptability and usage in roadway barrier systems.
This invention involves a substantially reconfigured or stabilized open section post. The unexpectedly strong synergisms of the characteristics found in the stabilized open section post not only address the above problems, but simultaneously obtain material savings. More particularly the synergisms may be described as follows.
One aspect of the present invention is that it has substantially redistributed material at critical locations as compared with conventional open section post configurations. This material redistribution has the effect of altering considerably the behavior of the post under combined axial, torsional, and bending loads, as compared with conventional steel posts, including open section posts.
Another aspect of the invention is that edge flanges that are formed within specific ranges of angles to adjacent flanges can provide additional edge strengthening for these innovative open section posts. The use of specific ratios of edge flange thickness to the radius between the edge flange and the adjacent flange may provide yet additional strength to the post section, while increasing its ability to absorb energy as a system component.
Yet another aspect of the invention is that the fracture resistance of the edge region is improved through specific combinations of edge flange characteristics such as length, radius, and angle to the adjacent flange. In embodiments that include bolt holes, the resistance of the bolt hole to fracture is substantially improved.
Another aspect of the present invention is that in some embodiments edge flanges or intermediate flanges between other flanges may have ribs. These ribs may be created as flutes or embossments in the axial direction or transverse to the axial direction, in order to increase the strength and buckling resistance of the post. It may be desirable in some instances to have two flutes cross each other in a transverse fashion. Also, the folded edge region itself may have at least one flute if desired.
Still another aspect of the present invention is that in some embodiments, embossing of the web or of specific flanges of the cross-section may be used in order to form reinforcing ribs that increase the overall resistance of the cross-section to lateral-torsional buckling as the post absorbs energy during a crash, or to increase the resistance of the post to nuisance damage and to local impacts during manufacturing, installation, and service. In such cases the embossing may protrude outwardly, away from the cross-section of the post, or inwardly, toward the interior of the cross-section of the post. Such embossing lends itself to roll form processing wherein the embossing is formed within the base sheet material from which the post is formed. However, other methods may also be used to form the reinforcing ribs other than embossing. These may include welding or bonding strips of like or different materials to the post during fabrication or installation of the post.
Another aspect of the present invention is that embossing of specific flanges is provided in order to further accommodate the placement of fasteners such as bolts. In these cases the embossing may also provide increased fracture resistance of the bolted connection. Embossments may be provided in the post cross-section so that two or more post sections may xe2x80x9cmatexe2x80x9d or interlock with one another in various ways. The embossed or fluted regions may also be roll formed and reinforced locally by adding additional material in order to achieve greater strength during installation or service, or to increase the resistance of the section to specific failure modes. Further, it may be desirable to have one flange made longer than the other in order that the wider flange can serve as a soil plate, in order to achieve manufacturing economies related to not having to weld on a separate soil plate section during fabrication.
An important aspect of the present invention is that the material redistribution required to obtain various collaborative effects is achieved in part by having specifically placed free edge portions, which are turned to define edge folds. The edge folds may be inturned or outturned. They may also be varied in size along the length of the post in order to achieve specific design objectives. The edge folds in one embodiment may comprise tubular beads or curls along the free edges which provide specific design synergisms and manufacturing economies that are consistent with the teachings of the present invention. Moreover, for this embodiment it is not just the presence of the tubular bead or curl that enables the substantial level of synergism, but the discovery of specific ratios of curl diameter to other post section dimensions that maximize these synergisms even to the extent of obtaining significant weight savings.
Another aspect of the present invention is that two sets of synergisms may be combined to make specific embodiments of the present invention even more successful. The first set of synergisms is directly related to the ratio of the diameter of the curl to the post section flange length. Each tubular bead may have cross-sectional dimensions which when combined in specific ratios with other post dimensions substantially maximizes the moment of inertia of the overall section about the section axes with a minimal use of material. Moreover, the tubular bead size specified by these same ratios may have the effect of altering the characteristic failure mode normally associated with the free edge stress concentration for conventional open section posts as described above. Finally, the cross-sectional dimensions of the tubular beads of the stabilized open section post make the novel post less sensitive to edge imperfections and damage because the blade edge may now be placed in a position of relatively benign stress levels so that imperfections or damage to the tube or edge fold region have to be on the order of size of the diameter of the fold or curl in order to have significant detrimental effect to the post section.
Another aspect of the present invention is that for some embodiments, having established the above ratios, a second set of synergisms was discovered by directly combining some of the above synergisms with specific ratios of the post""s cross-sectional web dimension to cross-sectional flange dimension. The compounding effect of the first set of synergisms with this additional set of ratios makes the stabilized open section post more resistant to torsion and edge buckling and thus avoids the problems that can plague deeper conventional open section posts using thinner gauge material. Additionally, these compounding synergisms make this particular embodiment unique in that stresses may now be more evenly distributed in the flanges, thus making the post more stable and less sensitive to dimensional imperfections.
Still another aspect of the present invention is that because of specific cooperative effects, some embodiments of the stabilized open section post demonstrate their uniqueness and efficiency in using thinner gauge material to accomplish the same tasks as conventional posts having much thicker sections.
Thus, when compared with conventional posts on the market today, some embodiments of the present stabilized open section post may use substantially thinner material while obtaining better resistance to crash loads on roadway barrier systems. Thus, even though additional slit width (the width of the sheet of material from which the post is made) is required to reposition needed material, the use of thinner gauge material more than offsets the additional slit width, thus bringing overall material savings as high as 18% in some instances.
Another aspect of the present invention is that for some embodiments the innovations in system configuration represent a potential cost savings for the manufacturer, since material cost is often a substantial portion of total manufacturing costs for roadway barrier hardware. The resulting unique and novel open section post may thus be very cost effective.
Another aspect of the present invention is the capability to strengthen the blade edge of open section posts against bending and buckling by redistributing material to the edges, which are typically regions of high stress during machine-aided installation as well as during a crash event. This redistribution can be further enhanced in the following way. In some embodiments the tubular bead is mounted on a turned (e.g. inturned) free edge portion. This enables the tubular bead and the turned free edge portion to act together synergistically. The result is a further stabilization of the cross-section of the post.
When the edge fold embodiment comprises a tubular bead for manufacturing process cost efficiency, preferably an open-section bead, the sheet metal edge fold is formed by turning the edge in an almost complete bend or curl, but the curl need not be closed at its outer edge, such as by welding. Such a closed section tubular bead would work equally well, at a somewhat higher manufacturing cost. This edge feature and other embodiments are discussed in more detail in the following paragraphs.
Another aspect of the present invention is that for some embodiments the edge folds are made by shaping the free edges or edge marginal portions of the flange cross-section into a non-circular, elliptical, or preferably (for manufacturing simplicity) circular, cross-sectional shape. As used herein, a circular cross-section is considered an embodiment of an elliptical cross-section and the term xe2x80x9celliptical cross-sectionxe2x80x9d includes a circular cross-section. The term xe2x80x9ccharacteristic diameterxe2x80x9d refers to a constant diameter in the case of a circle, while other elliptical shapes will have major and minor axes or diameters, with the major axis or diameter being the xe2x80x9ccharacteristic diameter.xe2x80x9d Even though some configurations of a slightly non-circular elliptical shape may be more desirable in some applications, the circular cross-section is generally preferable, because it is simpler to manufacture, while still achieving the desired benefits of edge folds to a significant degree.
For some specific embodiments it is important to contrast the edge curl approach against other possible edge treatment approaches by noting that the dimensional order of size effect related to imperfections or damages described above for the curl can not be achieved by simply folding the edge over, either once or multiple times, because in this case the characteristic dimension will be defined by the fold edge diameter and not by the length of overlap of the fold. This is because the overlap direction is transverse to the edge and quickly moves out of the peak stress region, and because the edge fold diameter defines the maximum distance over which the edge stresses may be effectively spread.
While the edge fold is illustrated as a tubular bead or curl, the edge fold may comprise polygon-shaped open or closed section edge folds. The edge fold shapes or designs may include non-circular, teardrop, elliptical or circular open-section tubular folds, and may be contrasted to tubular sections of rectangular cross-sectional shapes, including those with multiple-folded edges, and to open-section tubular shapes of softened corner polygon cross-sectional shapes in that the characteristic diameter will generally be defined in each of these other cases by the fold diameter or by the softened corner diameter nearest to the post section edge, as opposed to the overall diameter of the edge curl section. It may be noted that in this context, a teardrop, polygon or rectangular cross-section with very softened corners is in effect an imperfect ellipse or circle. In some instances, quasi-elliptical or quasi-circular cross-sections, imperfect ellipses, and imperfect circles, in the form of rectangular cross-sections with very softened corners may function adequately, but may also be more difficult to manufacture and may be less effective than a generally circular curl.
In some embodiments, an important additional edge strengthening and stabilizing capability is obtained by filling portions of the edge fold in cases where the edge fold cross-section is partially open. A simple example of this is inserting a round rod into an open circular shaped edge curl. In this case the rod may be held in place either by welding or bonding, or simply by providing an interference fit between the rod diameter and the inside diameter of the curl. This approach not only accomplishes edge strengthening, but also material redistribution in the post cross-section that can greatly increase the sectional properties such as the second moment of inertia of the post. As an example, this approach may be used to strengthen the post in the region of the xe2x80x9cground linexe2x80x9d where an installed post may protrude from the ground where it is installed. The ground line is commonly a region of high stresses during a crash event.
The resulting synergistic effect of the stabilized open section post""s material efficiency in obtaining the desired section moment of inertia, the alteration of the characteristic failure mode, the reduction in sensitivity to edge imperfections and damage, resistance to buckling and torsion as well as the ability to spread stresses more uniformly has the same degree of compounding advantage as the conventional I-beam and C-section post""s compounding disadvantage of low resistance to lateral-torsional buckling combined with sensitivity to relatively small edge or dimensional imperfections. Accordingly, the novel stabilized open section post of the instant invention provides a solution to the problems that the roadway safety post art has sought to overcome in conventional post configurations available hitherto. In summary, the stabilized open section post of the present invention may be uniquely designed to be compatible with substantially all standard roadway safety barriers, thereby significantly reducing the number of types of posts that manufacturers must carry in their inventories and package, to permit more stringent crash test requirements to be met, and to permit this to be done without major modification of other roadway safety hardware such as guardrails.
It may be noted that in some embodiments it may be desirable to supplement the strength of the edge region even further, with the addition of reinforcing fibers or wires, or even with strips of like or different material that may be attached to the post, such as by bonding, fastening, or welding. One example is the addition of rods to the post near the ground line in order to strengthen the post in this region of high stresses. The rod may be attached to the post, or it may be held in place by specific features of the post. This addition of material may be done for strengthening purposes, or as a means to modify the failure mode of the post further. Naturally, in such cases the manufacturing costs must be weighed along with the benefits obtained in order to establish the best possible product at the best possible cost.
In other embodiments the web or flanges of the post incorporate embossing or one or more flutes that form ribs that serve to strengthen them against buckling during mechanized installation or during service. In addition, in some cases, added beneficial strain hardening of the web or flange region material is obtained as the flutes or embossing are added. In some instances this strengthening may be supplemented or even replaced by local heat treatments, such as by plasma arc, laser, or flame related treatments. These may include the addition of special coatings, or the addition of material. Hydroforming technology may also be used in some cases, such as to form the edge curls, embossing, flanges, or ribs. Variable base material thickness in the post may also be used within the teachings of the present invention.
Another aspect of the present invention is that it has provisions for a modified end, such as to make that end behave somewhat like a blade with tapered edges that permit it to act like a wedge as it is forced into the ground such as by mechanical means during installation. Local heating of these followed by quenching may be used in order to strengthen the base material locally, in order to make the blade region stronger.
The following description of the present invention may incorporate dimensions which are representative of the dimensions which will be appropriate for most commonly found roadway barrier systems. Recitation of these dimensions is not intended to be limiting, except to the extent that the dimensions reflect relative ratios between the sizes of various elements of the invention, as will be explained where appropriate.